More than half of credit-card holders don’t know if their credit card charges a foreign transaction fee, according to a survey performed by personal finance website WalletHub. And that can be one costly mistake.

Foreign transaction fees are charged by your card issuer—so, Chase or Wells Fargo, for example—on every purchase made outside the U.S., or on domestic purchases made through companies based outside the U.S. (For example, if you book a hotel room in London through the hotel’s website instead of through something like Expedia or Kayak, you’ll likely pay a foreign transaction fee.) They’re typically around three percent per swipe.

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So if you took a week-long vacation to Europe, let’s say, and spent $1,000 on your credit card, you’d owe an additional $30 in fees. Spend a bit more, and it could become more expensive than an annual fee or easily overshadow cashback and other rewards. (On top of that, you’ll likely pay a conversion fee if you take cash out of an ATM.)

A separate report from CompareCards.com, also released Tuesday, found that nearly half of all cards charge the fee (WalletHub found that 72 percent charge one). And while both banks and credit unions assess foreign transaction fees, bank fees are typically higher: While credit unions charge a fee of 1.15 percent on average, bank-issued credit cards assess a 2.97 percent fee. To find out if your card has one, check the “fees” section of your card’s terms and conditions.

How to Avoid Foreign Transaction Fees

Many cards that don’t have foreign transaction fees come with an annual fee. “On average, the cards in our study with no foreign transaction fee charged an annual fee of $86.76, versus an average of $14.99 annual fee among cards that assess foreign transaction fees,” CompareCards reports. Still, if you spend a decent amount on your trip, you’ll easily cancel out the fee with your savings, plus other perks those cards tend to offer, like trip cancellation insurance, lost luggage coverage, access to lounges and Global Entry.

That said, there’s no shortage of cards that charge neither an annual fee nor foreign transaction fees. CompareCards found that 27 cards don’t charge either fee, including the Chase Amazon Prime cards and all Discover and Capital One cards.

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Another thing to keep in mind: Your card’s payment processor (so, Visa, for example) will also charge currency conversion fees. Visa and MasterCard charge one percent of each dollar you spend (depending on which country you’re in). NerdWallet notes that these fees often aren’t listed individually on your statement like foreign transaction fees, and not every bank passes them on to you, the consumer.

“It’s not always clear from your credit card’s disclosures how much currency conversion fees will cost you,” writes NerdWallet. “Many consumers find that they have to compare the totals on their receipts to the transaction costs that appear on their statements or contact their card issuers to find out how much they’re being charged.”

If you’re being charged more than a one percent conversion fee, shop around for a better card. You can also ask your issuer what currency conversion rate it’s using before your trip so you have a better idea what you’ll be charged. That said, WalletHub notes that using a credit card will likely get you the best conversion rate—converting at a bank abroad or at the airport will likely be costlier. And using a credit card is still going to be safer than filling your suitcase with cash.